fix backtrace printed when cannot get lock.
change lock error handling code so exceptions have useful info and
exception handling in one place.
add test case for when cannot get lock.
#!/bin/sh
hg init a
cd a
echo a > foo
hg add foo
hg commit -m 1 -d "1000000 0"
rm foo
hg remove foo
hg commit -m 2 -d "1000000 0"
hg export 0
hg export 1
hg log -p -r 0
hg log -p -r 1
cd ..
hg clone a b